In oilfield exploration and production operations, floating vessels, such as semi-submersibles (“semis”), are commonly used for various offshore tasks, including, but not limited to, drilling rigs, safety platforms, and heavy lift cranes. For example, in an offshore environment with water depth greater than 120 meters, semis are used in because a fixed structure is not practical to build, maintain, or support in such great water depths. Furthermore, semis are advantageous over other floating vessels, such as drillships, as drillships are unstable in rough offshore conditions having large waves and strong tidal forces. As would be understood by one having ordinary skill, offshore semis are not limited to the aforementioned water depths described in the above example.
Referring to FIG. 1, a side view of a semi-submersible 113 in a typical marine environment is shown. A deck 101 sits above the surface of water 111. The deck 101 is typically used for drilling, production, or other operations and therefore operating equipment, personnel, and operation gear may be disposed thereon. The deck 101 may be supported by one or more support columns. As shown in this example, the deck 101 is disposed on support columns 103A and 103B and is therefore kept away from any large waves at the surface of the water 111. Support columns 103A and 103B are used to support the deck 101, but may also serve as storage. In addition, support columns 103A and 103B may be ballasted. A pontoon base 105 has the support columns 103A and 103B disposed thereon. The pontoon base 105 may be substantially rectangular in shape from a side view perspective, a plan view perspective, or both.
The semi-submersible 113 obtains buoyancy from ballasted pontoons or ballasted columns. As such, the ballasted structure(s) (ballasted pontoons or ballasted columns or both) may be filled with water or any other ballasting material (ballasting) or may release water or any other ballasting material (deballasting) to stabilize the semi-submersible 113. As shown, the semi-submersible 113 is anchored to the seabed 109 by anchor lines 107A and 107B. The anchor lines 107A and 107B may be wires, chains, or any other anchoring device known in the art that would keep the semi-submersible in a proper position with respect to the seabed 109. Furthermore, anchor lines may not be limited to only two lines as shown in this example. The semi-submersible 113 may be anchored by any number of anchor lines.
Alternatively, for use in marine environments with a shallower water depth, the semi-submersible 113 may be adapted to be disposed on seabed 109 without the use of anchor lines 107A and 107B. In this case, pontoon base 105 may be disposed on the seabed 109 and may be affixed to the seabed 109 using an affixing unit (not shown) to affix the pontoon base 105 and ultimately, the semi-submersible 113, to the seabed 109.